To assess the effects of a probiotic upon oral health indices in adolescents and to establish relationships between these indices and dietary habits and oral hygiene. Twenty-seven adolescents between 12 and 18 years of age were randomized into two groups. The study group received tablets containing Lactobacillus reuteri DSM 17938/ATCC 5289 for 28 days, while the control group received tablets without any bacteria. Streptococcus mutans, Lactobacillus sp., and salivary pH were assessed at baseline and at 7, 14, 21, 28, and 45 days. The plaque, gingivitis, and bleeding indices were recorded at baseline and at 14, 28, and 45 days. Dietary and oral hygiene habits were also evaluated by means of a questionnaire. A less marked rise in S. mutans was recorded in the study group. Improvements were observed in terms of plaque, gingivitis, and bleeding, though statistical significance was not reached. Oral pH increased in the study group, though not to a significant degree. Poorer eating habits were significantly correlated to increased plaque. The study parameters decreased with the two strains of L. reuteri DSM 17938 and ATCC PTA 5289, though the results failed to reach statistical.